1. Field of the Invention
The present invention is related to a fiber sensors uniformly embedded in a composite material such that discontinuities at the fiber/composite material interface are eliminated, or at least greatly minimized. The invention also relates to a sensor which may have a preferred orientation, and to a method for easily identifying that orientation to a user of the sensor. In particular, however, it is anticipated that the invention will find its greatest utility in providing substantially long lengths of a fiber, a wire, or some combination of these, and potentially articles such as silicon chips or another device, in an overlaying member which surrounds the fiber and which is itself easy to fabricate into a mechanical structure.
This invention also relates to a method for placing a fiber, a wire, or some combination of these into a composite material such that resin pooling around the fiber is substantially reduced or eliminated thereby providing a member exhibiting generally uniform properties along a radial direction and along a longitudinal direction with respect to the fiber.
2. Prior Art
There is increasing interest and use of composite materials, for many years. Typically, such composite materials comprise various layers or plies of a fiber, e.g. glass fiber, carbon fiber, or some other fiber, bonded together by a resin or polymer.
For various reasons, composite materials are gaining wide acceptance in many industries where the ability to accurately monitor component structural integrity or surrounding environmental conditions is vital. Furthermore, with the development of fibers having embedded anisotropic mechanical features it is now possible to monitor several physical parameters simultaneously. See U.S. Pat. Nos. 5,828,059 and 5,591,965.
Monitoring techniques have been proposed, using a network of optical fibers embedded into the structure of a component and forming an integral part of it, and such techniques potentially have a number of advantages. In principle, optical fiber sensing technology can be used for a number of purposes: e.g. impact detection and location; delamination and microcrack detection and location; strain and deformation mapping.
It is known to embed optical fibers into a composite material. One proposal can be found in U.S. Pat. No. 4,581,527 (Crane et al) disclosing a damage assessment system using a three-dimensional grid of embedded optical fibers. U.S. Pat. Nos. 4,854,706, 5,210,499 and 5,367,376 discuss embedding optical fibers in resins or resin impregnated tapes. Furthermore, U.S. Pat. No. 4,854,706 discusses orienting the optical fiber within the resin composite. However, none of the references teach a means for consolidating a fiber into a second material such that the fiber is embedded into the matrix of that second material without surrounding areas of discontinuity, such as pockets of resin collecting around the fiber at the fiber/matrix interface.
What is needed, therefore, is a structure containing a length of the oriented fiber and comprising the same or a similar, compatible prepreg tow composite as the member to which the sensor is to be applied. Also needed is a simple method for producing the desired lengths of tow structure such that no particular care would be required by an assembler to quickly position and xe2x80x9clay upxe2x80x9d the tow structure on, or in, the member to which the fiber sensor is to be incorporated.
To address this need, the Applicants herein disclose a method which begins with the device described and disclosed in co-pending U.S. patent application Ser. No. 09/603,460. The utility of that device is now extended to comprehend any fiber incorporated into a second matrix material such that it is consolidated without discontinuities or other non-uniformities along its length. In particular, it is disclosed that a fiber may be laid up in a composite structure such that resin xe2x80x9cpoolingxe2x80x9d at the fiber resin matrix interface is eliminated or substantially reduced.
It is therefore an object of this invention to provide a means for placing an optical fiber into a prepreg tow member which is to be further assembled into a composite structure.
Yet another object of this invention is to provide a method for embedding a fiber within a prepreg tow such that resin is prevented from xe2x80x9cpoolingxe2x80x9d or forming preferentially zones or pockets at the surface of the fiber.
Additional objects and advantages of this invention will become apparent to those skilled in these arts as the following specification and claims is reviewed.